Where low voltage subsets are used the memory of the dialler circuit is conventionally maintained when the subset is in the ON-HOOK mode by an internal battery in the form of a dry cell usually of the lithium type. The battery is disconnected from the dialler circuit's power terminals when the subset is brought into the OFF-HOOK mode, the power terminals then being connected to the exchange battery via the subset's operated hook switch.
It is imperative that the internal battery terminals are electrically isolated from the exchange battery in order to avoid detrimental charging of the internal battery where the exchange battery voltage exceeds the internal battery voltage, and, where the internal battery voltage exceeds the exchange battery voltage, to avoid discharging of the internal battery.
A problem arises when a low voltage subset is connected in a parallel arrangement with one or more other subsets in that, in the event that the low voltage subset is in the OFF-HOOK mode and its dialler circuit's memory is being powered by the exchange battery, and another subset is brought into the OFF-HOOK mode, the voltage at the line terminals of the subsets drops to a level insufficient to maintain the memory in the dialler circuit.